The sun radiates about a kilowatt of energy on to each square meter of the surface of the earth when incident thereon in perpendicular relationship. A large number of different methods of and apparatuses for using that solar energy radiated by the sun is known.
In photovoltaics sunlight is directly converted into electric current. It will be noted however that the costs of that method are very high, at the present time a square meter of photovoltaic solar cells costs about 700.00. The energy efficiency of the method is about 20%.
An alternative approach involves using energy radiated by the sun for producing heat. Some methods and apparatuses are known in that respect, in which sunlight is concentrated by means of trackable mirrors.
The heat to be achieved in that way serves for example for heating an oil or for heating a liquid salt. The oil or salt heats water to produce steam. The steam produced is used to drive a steam turbine for power generation by means of a power generator. That approach is technologically viable but it is found to be comparatively complicated and expensive.
A further approach involves the provision of parabolic mirror installations in which a sufficiently large, closed mirror surface is used to track the sun. That tracking operation is effected by pivoting and inclining the parabolic mirror installation so that actuation in respect of two axes is necessary. As a consequence of the large mirror area such an installation is generally high in weight so that it is only with corresponding difficulties that it can be directed towards the sun with an adequate degree of accuracy. The focal point of the system moves with the position of the sun, which causes difficulties in terms of using the focused rays of the sun as thermal energy. An example of such a system is a ‘solar dish’, as is described for example at http://www.solarpaces.org/resources/technologies.html.
Simpler tracking can be achieved with trough-type mirror systems as they only have to be moved about one axis. The trough-type mirrors heat a tube which extends in its focal line. A problem arises in terms of technical implementation in that long tube systems have to be adjusted, heated and protected from heat losses, which in turn makes the installations expensive. An example of such a system is the Andasol project (http://ww.solarmillennium.de/).
In the so-called ‘solar tower’ concept a large number of mirrors is directed on to a common point at the tip of the ‘solar tower’. In that case in turn each of the mirrors must be individually controlled about two axes, which makes the method expensive in consideration of the accuracy required in that respect (see for example http://www.bmu.de/pressemitteilungen/pressemitteilungen_ab—22112005/pm/37405.p hp). The price of such an installation is about 1100.00 per square meter of mirror area and thus, with an energy efficiency of about 10%, is even higher than the price for the photovoltaic system.
A need for tracking of mirror systems for the concentration of sunlight arises out of the fact that, from the point of view of the earth, the sun apparently follows a flat circular orbit around the earth. The plane of that circular orbit can be described in that case by a unit vector which is perpendicular to that plane. As a consequence of the inclination of the axis of the earth with respect to the orbit the direction of that unit vector changes in the course of a year, which manifests itself in the fact that the midday height of the sun, that is to say the apparent height of the sun above the horizon, varies in the course of the year.
The known apparatuses for and methods of using solar energy are structurally complicated and expensive and in part costly to provide and operate.